Help with GGG Bronx Cheer build?

[Gristlepig]

Hey folks,
I've lurked around for a while, but this is my first post. Let me know if this is miscategorized or whatnot.

So I recently built up a Bronx Cheer pedal using the board and instructions from GGG and it's not so cheery as it might be.

The first time I powered it up and switched on, the led blew. And though it passes signal when the effect is engaged, it just sounds like a crappy, weak fuzz.

Initially, neither the volume knob nor the hi-lo switch had any effect on the sound. I just today grounded the 3rd lug of the volume pot to the board (omitted in the ggg diagram) and now the volum knob works, but the circuit still sounds like turds.

I used an MPSA13 transistor from Small Bear and my voltage readings are:  Collector 8.73v, Base 8.45v, Emitter 7.58v

I dunno where to go from here. I've successfully built and/or debugged about a dozen dirtboxes, but this one has got me stumped.

Any help or guidance is greatly appreciated!

[rutabaga bob]

Welcome to the forum!  The LED blowing sounds like it didn't have the limiting resistor wired right, or maybe not a big enough value.  I have built a couple, and, though I liked the unusual sound of the fuzz, they didn't give the 'razzberry' tone I was expecting.  In my latest, I did some experimenting, and found that using a .022 capacitor on the switch to ground works much better than the .01 specified.  I also experimented with a .033 and a .068, but still like the .022 best.  As a side note, did you wire in the transformer?  Also, if you have long runs of input/output wires, use shielded cable (RG172).  I have had to go back in and replace the in/out wires, as I was getting a horrible whine.  Lots of luck on your build!  Larry

Edit:  When you're ready to try it, turn up the effect, use the bridge pickup, and roll the guitar volume down to 5 or 6.  Sometimes it helps to swap out transistors, if you have more than one mpsa13 to choose from.  As an aside, on the GGG build, the pot is shown grounded at the jack instead of the board, but it doesn't matter.

[Earthscum]

I used an MPSA13 transistor from Small Bear and my voltage readings are:  Collector 8.73v, Base 8.45v, Emitter 7.58v

You are measuring voltage from ground to Collector, Base, and Emitter? If so, I suspect your power is wired backwards. If not, then your voltage readings should be more in the ballpark of C - 1.15V, B - 0.28V, E - 0V. That should, however be the Collector being at 0V, though. Doing the voltages backwards, you may have confused C and E (I still have to edit posts occasionally concerning Collector or Emitter  ).

So, to wrap up, if you measure from the leg closest to the edge of the board to the middle leg, you should have about 1.2V, or 2 B-E diode drops. The base is held 2 diode drops (about 1.2V) below the Collector, the leg farthest from the edge. It should read about 1.8V. I'll have to pop one on the board to double check this, though. If you're misreading your transistor legs, then I suspect you are dumping power across the LED, and it's not leaving enough to swing the diodes in the feedback.

[Earthscum]

I have voltages of 1.05(B) and 1.08(C)

ETA: Playing around a bit, I found that if you use .01uF's instead of .001uF's in the feedback, and putting yet one more .01uF from Base to Ground will make it spit, sputter, and meow at you (all in the same pluck... it's beautiful). If you notice the highs aren't getting the reaction they deserve, decrease the cap from base to ground to .0068 or maybe even .0033, but the smaller you get, the less reaction you get from the circuit.

For the "Hi-Low" switch, the best I could come up with is maybe .047uF. Basically tames it down a bit.